Load distribution in elongated members of vertically moored drilling vessel

ABSTRACT

THIS INVENTION RELATES TO A SYSTEM FOR MOORING A FLOATING STRUCTURE TO THE OCEAN FLOOR. SPECIAL BUOYANCY CHAMBERS SUPPORT THIS STRUCTURE, INCLUDING A WORK DECK ABOVE THE BODY OF WATER. THE STRUCTURE IS CONNECTED TO ANCHORS IN THE FLOOR OF THE BODY OF WATER BY ELONGATED MEMBERS SUCH AS CABLES AND PIPES HAVING DIFFERENT ELONGATIONS PER UNIT OF STRESS. BECAUSE OF WAVE ACTION, THE FLOATING PLATFORM IS SUBJECT TO VERTICAL HEAVE FORCES. BECAUSE OF SUCH INCREASED TENSION, THE CABLE AND PIPE RISERS ARE BOTH ELONGATED BUT WITH THE CABLE UNDERGOING THE MOST ELONGATION. HENCE, THE INFLUENCE OF VERTICAL HEAVE FORCES IS FELT ALMOST ENTIRELY BY THE PIPE AND NOT BY THE CABLE. IN ACCORDANCE   WITH THIS INVENTION A LOAD DISTRIBUTION MEANS IS PROVIDED TO CONNECT BOTH THE CABLE AND THE RISER PIPE TO THE FLOATING STRUCTURE. IN PREFERRED EMBODIMENT THE UPPER END OF THE RISER PIPE AND THE UPPER END OF THE CABLE ARE CONNECTED TO OPPOSITE ENDS OF A FLEXIBLE MEMBER WHICH GOES OVER A PULLEY WHICH IS SUPPORTED FROM THE SUPERSTRUCTURE. THE CABLE AND PULLEY SYSTEM PROPORTIONS THE TOTAL TENSION BETWEEN THE PIPE AND THE CABLE ACCORDING TO A DESIRED RATIO.

Feb.f2 1971 K. A; BLEIQRARN ALLY KENNETH A. BLENKARN .IN ELONGATED MEMBERS 0F VERTIC MOORED DRILLING. VESSEL Filed Feb. 10, *1969 LOAD DI STR IBUTION INVENTOR.

y}. x9 12 M022 ATTORNEY United States Patent Office 3,559,411 Patented Feb. 2, 1971 LOAD DISTRIBUTION IN ELONGATED MEMBERS OF VERTICALLY MOORED DRILLING VESSEL Kenneth A. Blenkarn, Tulsa, Okla., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Feb. 10, 1969, Ser. No. 797,861 Int. Cl. E02d 21/00 US. Cl. 6146.5 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a system for mooring a floating structure to the ocean floor. Special buoyancy chambers support this structure, including a work deck above the body of water. The structure is connected to anchors in the floor of the body of water by elongated members such as cables and pipes having different elongations per unit of stress. Because of wave action, the floating platform is subject to vertical heave forces. Because of such increased tension, the cable and pipe risers are both elongated but with the cable undergoing the most elongation. Hence, the influence of vertical heave forces is felt almost entirely by the pipe and not by the cable. In accordance with this invention a load distribution means is provided to connect both the cable and the riser pipe to the floating structure. In a preferred embodiment the upper end of the riser pipe and the upper end of the cable are connected to opposite ends of a flexible member which goes over a pulley which is supported from a superstructure. The cable and pulley system proportions the total tension between the pipe and the cable according to a desired ratio.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a structure floating on a body of water. More particularly, the invention relates to a system for anchoring such floating structure to the floor of the body of water. It relates to a mooring system in which the mooring is accomplished by both essentially vertical cables and pipe risers. It relates especially to a system whereby the total load carried by the pipes and the cables is distributed according to a desired ratio even under heaving of the floating structure.

(2) Setting of the invention In recent years there has been considerable attention attracted to the drilling and production of wells located in water. Wells may be drilled in the ocean floor from either fixed platforms in relatively shallow water or from floating structures or vessels in deeper water.

In recent years there has been attention directed toward many different kinds of floating structure which for the most part have been maintained on station by conventional spread catenary mooring lines or by propulsion thruster units. One scheme recently receiving attention is employed in the so-called vertically moored platform. One such platform is described in US. Pat. 3,154,039, issued Oct. 27, 1964. A key feature of the disclosure in the patent is that the floating platform is connected to an anchor base only by elongated parallel members. These elongated members are held in tension by excess buoyancy of the platform. This feature offers a remedy for one of the major problems arising in the conduct of drilling, or like operations, from a floating structure. This major problem is that ordinary type barges or vessels response to ocean waves may exert substantial amounts of vertical heave or roll motion. Such motion significantly hinders drilling operations. Motion difliculty is alleviated to a degree by use of the so-called semisubmersible vessels in which floatation buoyancy is provided by long, slender, vertical bottles or tanks. This design suffers the inconvenience that, if carried to the logical extreme of having very little water plane area, the unit would have onl a small margin of stability, requiring careful re-ballasting to offset changes in vertical loads such as drilling hook load (e.g., when pulling drill pipe, etc.) or changes in weight of supplies. Some of these problems are eliminated or at least reduced in the vertically moored platforms. Being subjected to tension, the elongated parallel members of the vertically moored platform are extensible only to a small extent and thus restrain the platform to move pri marily in a horizontal direction. This virtually eliminates heave and roll motion. In vertically moored structure heretofore considered, exceptionally strong mooring would be required to resist the vertical forces which might be imposed on the structure by the horizontal motion of passing waves. My co-pending application 754,628, filed Aug. 22, 1968, describes a system to eliminate or minimize the vertical forces imposed on a floating structure, such as those caused by passing waves.

In these prior vertically moored structures, all of the parallel members have been of the same type, i.e., all types having similar elongations due to stresses.

BRIEF DESCRIPTION OF THE INVENTION In some cases it may be desirable to moor the vertically moored structures discussed above by use of intermixed cables and riser pipes. Cables and riser pipe will ordinarily have different elongations for the same stress and conversely, for a given change of length, the pipe would experience relatively greater change in load. Then a vertical heave on the vessel will cause most of the additional stress caused by such vertical heave to be taken up by the pipe. In accordance with my invention I connect the pipe and cables through load distribution means to the floating structure. In a preferred embodiment this load distribution means between the pipe and the cable takes the form of a pulley arrangement in which the axis of the pulley is supported from the floating structure and the cable or other flexible means extends over the pulley. Opposite ends of the pulley cable are connected to the cable and the riser pipe, respectively. If the cable and riser pipe are subjected to an increase in length, they both would tend to elongate, with the cable undergoing most elongation. However, with my pulley arrangement the ratio of the tension between the two is maintained constant. The difference in elongation is compensated by a motion of the pulley system.

DRAWINGS Various objects and a better understanding of the invention can be had from the following description taken together with the drawings in which:

FIG. 1 illustrates one leg of a vertically moored platform which is anchored to the ocean floor by a cable and a pipe;

FIG. 2 illustrates a modification of the embodiment of FIG. 1 showing another means for apportioning the stress between the cable and the pipe;

FIG. 3 illustrates a modification showing distribution of load between a pipe and a plurality of cables.

In FIG. 1 a working deck 10 is supported above the surface 12 of the body of water 14 by a plurality of vertical, bottle-shaped pontoons or buoyancy chambers 16. There is only one such buoyancy chamber 16 shown but there are usually three or four buoyancy chambers. Mounted on deck 10 is a derrick 18 which is used for drilling and workover operations. As shown, deck 10 and buoyancy chamber 16 amount to a vertically moored platform inasmuch as they are anchored to the ocean floor only by vertically parallel, elongated members. For a more complete discussion of vertically moored platforms, attention is further directed to my co-pending application S.N. 754,628, Vertically Moored Platforms, filed Aug. 22, 1968.

In the prior art, the elongated members which hold the floating structure to the ocean are of the same type, that is, all are pipe or all are cable. However, in some situations, I have found that sometimes it may be desirable that both cable andpipe be used in mooring. As mentioned above, this presents a problem due to different elongation of the pipe and cable under the stress induced by heaving of the floating structure. However, as will be seen, I provide means for maintaining a desired proportion of the load distributed between the cable and. the pipe under such conditions. However, before describing such load distribution means in detail, a brief consideration of the stress variations in steel pipe and steel cable for a given elongation of each will be helpful.

Consider a riser pipe of length L which, because of heaving forces, is subjected to increase of length AL. Then the stress in the pipe, initially at some value (1 will undergo an increase Aa according to the relation:

in which E is the elastic modulus of the pipe. The relative change in stress is expressed as:

& L 0'9 The comparable expressions for a cable is expressed as:

meaa Ao' AL an =2,000 T 4) L A2 an 200 L (5) Thus, for such typical design conditions, the relative stress variation in the pipe is times that in the cable.

Shown in FIG. 1 for anchoring buoyancy chamber 16 to anchors is a riser cable 20 and a riser pipe 22. These two members are mounted essentially vertical and parallel to each other. They are anchored in the bottom 24 by any known method, such as being embedded. in a cement block 26 set in the bottom 24. Riser cable 20 and pipe 22 extend up through vertical passages 21 and 23, respectively, in buoyancy chamber 16.

The upper ends of riser cable 20 and pipe 22 are connected to the upper structure 28 of buoyancy chamber 16 in a special manner. A plurality of pulleys 30 are supported on horizontal axis 32 which is supported from upper structure 28. The upper end of pipe 22 is provided with an enlarged cross member 34. Similarly, the upper 7 end of riser cable 20 is connected to a cross bar 36. A plurality of flexible lines or pulley cables 38 are connected at one end to cross member 34 of pipe 22 and extend over pulley 30. The other ends of lines 38 are connected to cross member 36 of riser cable 20. By this arrangement any difference in elongation in pipe 22 and riser cable 20 due to heaving of the platform 10 is compensated for by movement of the pulley and line arrangement so that each carries the same load.

By proper selection of the number of riser cables, pipes, and pulley cables connecting them, any desired proportion of load distribution can readily be made. For example, in FIG. 3 there are two riser cables 20A and 20B and pipe 22A. The upper end of pipe 22 has two pulley cables connected thereto. One pulley cable 42 goes over a first pulley 40 to cable 20B. The second pulley cable 44 goes over pulley 46 and. is connected to the upper end of riser cable 20A. In this arrangement, pipe 22A has the same load as the total load carried by riser cables 20A and 20B. By making the proper selection of pulleys and connecting lines or block and tackle, or wheel and axis, or other load proportioning means, any selected load distribution can be obtained.

FIG. 2 shows another means of obtaining a desired load distribution between a riser cable and pipe in a mooring system. Shown in FIG. 2 is riser cable 50 and pipe 52. These each extend up through vertical passages 54 and 56, respectively, within float 16. A lever means is provided at the upper portion of extension 28 to support the pipe and riser cable. This includes a horizontal lever 58 having a fulcrum 60 supported from the structure 28 itself. Riser cable 50 is connected at pivot point 62 at one end of lever 58 and pipe 52 is connected at pivot point 64 on the other end of lever 58. These connections 62 and 64 are pivotal connections such as horizontal axis. The distance from the fulcrum 60 to point 62 and to point 64 of course determines the load distribution between riser cable 50 and pipe 52.

The pulley and cable arrangement or other load distributing means in each embodiment is shown as being in the leg above the water level. This permits repair or replacement of the pulley cables or other parts as they become worn. In the embodiment of FIG. 1, one pulley cable 38 can be taken off and replaced at a time. This replacement would normally occur during calm Weather so that excessive forces would not be encountered.

The above embodiments have been described with a certain amount of detail; however, it is to be understood that various modifications can be made therefrom without departing from the spirit or scope of the invention.

I claim:

1. A floating structure for use in a body of water which comprisesn a first elongated member connecting the floating struc-- ture to an anchor means on the bottom of said body of water;

a second elongated member connecting the floating structure to an anchor means on the bottom of said body of water, the elongation per unit stress being much greater for said second elongated member than for said. first elongated member;

load distribution means connecting said first and said second elongated member to said floating structure for distributing a variable load in a fixed proportion between said first elongated member and said second elongated member.

2. An apparatus as defined in claim 1 in which said first elongated member is a Isier cable and said second elongated member is a pipe which are essentially parallel to each other, there being no connecting means between said floating structure and said anchor means other than said parallel elongated members.

3. An apparatus as defined in claim 2 in which said load distribution means includes a pulley whose axis is supported from said floating structure, a pulley cable extending over said pulley, one end of said pulley cable connected to said riser cable and the other end connected to said pipe.

4. An apparatus as defined in claim 2 in which said load distribution means includes a beam, a pivot intermediate the end of said beam, means to support said pivot from said floating structure;

means connecting one end of said beam to said riser cable and means connecting the other end of said beam to said pipe.

5. An apparatus as defined in claim 1 in which said load distribution is located above the surface of said body of water.

6. A floating structure for use in a body of water which comprises:

a first elongated member connecting the floating structure to an anchor means on the bottom of said body of water;

a second elongated member connecting the floating structure to an anchor means on the bottom of said body of water, the elongation per unit stress being much greater for said second elongated member than for said first elongated member;

load distribution means including a pulley whose axis is supported from said floating structure, a pulley cable extending over said pulley, one end of said pulley cable connected to said riser cable and the other end connected to said pipe.

7. A floating structure for use in a body of water which comprises:

a first elongated member connecting the floating structure to an anchor means on the bottom of said body of water;

a second elongated member connecting the floating structure to an anchor means on the bottom of said body of water, the elongation per unit stress being much greater for said second elongated member than for said first elongated member;

a load distribution means including a beam, a pivot intermediate the end of said beam, means to support said pivot from said floating structure;

means connecting one end of said beam to said riser cable and means connecting the other endof said means to said pipe.

References Cited UNITED STATES PATENTS 2,777,669 1/1957 Willis et al 61--46.5 3,407,767 10/1968 McClintock et al. 114.5

JACOB SHAPIRO, Primary Examiner US. Cl. X.R. 

